In recent years a great deal of money and effort has been expended on developing automation systems for important industrial processes. However, at present a large number of industrial chemical or biological processes cannot be subjected to process control for various reasons. One of the major reasons is the fact that some processes are extremely difficult or even impossible to monitor in such a way that the information gathered can be used for on-line control of the process, since sensing systems for measuring physical process parameters often do not yield results which adequately reflect the actual state of the process. Hitherto known analytical methods for the monitoring of process parameters are thus generally inadequate. If these processes could be adequately monitored, their optimization would be facilitated, which in turn would result in economic and environmental benefits in the form of, for example, higher yields, energy savings, decreased pollution, etc.
Automation systems necessarily include some kind of device and method for monitoring one or more process parameters so as to obtain relevant and adequate information about the actual state of the system. For on-line control purposes it is essential that the device employed give rapid, reliable and reproducible results, and it should preferably be relatively simple to employ, inexpensive, and constructed in a form which is as compact as possible.
A likely candidate for a method for monitoring in this manner is the analytical method of Flow Injection Analysis (FIA). FIA is based on a combination of the following features: injection of a well-defined volume of sample into a non-segmented, continuously flowing carrier stream of reagent, controlled dispersion of the injected sample zone during its transport from the point of injection to the point of detection, and reproducible timing of all events. In recent years, FIA has developed from an approach for merely rapidly conducting serial assays into a novel concept in solution handling in analytical chemistry and a diagnostic tool to be exploited for general analytical studies. These further developments include a number of variations, for example stopped-flow FIA and flow-reversal FIA. In stopped-flow FIA, the flow is stopped at intervals for an appropriate period of time with the dual purpose of increasing the residence time (which increases the yield of the measured component and thus increases the sensitivity of the measurement) and measuring a reaction rate which serves as the basis for the analytical readout. In flow-reversal FIA, discontinuous passage of the sample plug through the detector in an open flow system is carried out by repeated reversals of the flow; in these reversals, the whole plug is not allowed to pass through the detector, but only a preselected zone of the plug is "sampled", so that the inversion of the cycles takes place within one FIA peak.
Today, traditional FIA, as well as methods developed on the basis thereof, are widely used in analytical laboratories for the automation of wet chemical analyses. Usually, these methods are not applied to process use; however, in a few cases FIA has been used for the monitoring of chemical reaction processes.
The use of FIA for process control purposes is more complicated than its use in the automation of routine laboratory analysis. The complications are particularly associated with the attainment of a sample suited to injection in a FIA system. Withdrawal of a sample from a reaction vessel can give rise to considerable problems such as clogging, change in composition as the result of reaction taking place in the sampling line and delays. It is therefore desirable to avoid the use of sample lines and to use instead equipment which can be introduced directly into the reaction vessel.
An important industrial chemical process which, owing to a lack of adequate monitoring methods, generally speaking is not subjected to on-line process control at present is the process (sometimes known as the azo process) for production of poorly soluble azo pigments, which are a commercially very important class of chemical substances. Azo compounds in general are a class of strongly coloured compounds. They can be intensely yellow, orange, red, blue or green, depending upon the exact structure of the molecule. Due to this property, azo compounds are of tremendous importance as colourants, especially as dyes and pigments.
By the term "colourant" is meant a (usually strongly) coloured chemical compound.
By the term "dye" is meant a colourant which is substantially soluble in the medium in which it is to be used.
By the term "pigment" is meant a colourant which is substantially insoluble in the medium in which it is to be used.
Azo compounds of the general formula Ar--N.dbd.N--Ar', where Ar and Ar' independently denote an aromatic group, can be obtained by a coupling reaction in which diazonium salts react with certain aromatic compounds, viz. nucleophilic compounds, in a electrophilic substitution reaction: EQU ArN.sub.2.sup.+ +Ar'H.fwdarw.Ar--N.dbd.N--Ar'+H.sup.+
The nucleophilic reactant is denoted the coupling component. Suitable coupling components for the formation of azo colourants are aromatic compounds with nucleophilic centres at the aromatic ring, especially naphthols or compounds bearing methylene groups which can undergo enolisation. In a coupling reaction, naphthols react as naphtholates, and the methylene-containing compounds react as enolates.
In order to obtain substantially optimum reaction conditions, pH should be kept constant by the addition of base or buffer, since free acid is formed in the coupling reaction. However, strongly alkaline reaction conditions must be avoided, since the diazonium compound under such conditions reacts to form a trans-diazotate which cannot function as a reactant in a coupling reaction and which decomposes leading to contamination of the reaction mixture. Furthermore, if coupling proceeds too slowly because of unfavourable conditions, phenol formation and/or formation of other undesired products may become the major reactions. In such cases, the phenol formed from the diazonium salt can itself undergo coupling; even a relatively small amount of this undesired coupling product can contaminate the desired material, usually a colourant whose colour should be as pure as possible, to such an extent that the product is worthless. Phenols, naphthols and enols are preferably coupled under mildly acidic to mildly alkaline conditions. Phenols couple fastest in mildly alkaline solutions, and amines couple fastest in mildly acidic solutions. The coupling reaction usually takes place in water.
In the batch-wise production af azo compounds in an industrial process, the actual concentration in the chemical reaction medium of the diazonium reactant which takes part in the coupling reaction is of utmost importance. As is apparent from the above discussion, if the concentration is too high, this may lead to the formation of undesired side products which may discolour the desired product. Furthermore, the formation of undesired side products decreases the actual yield of the reaction, which leads to higher production costs. The rate of reaction also has a significant impact on the degree of (over)saturation of the chemical reaction medium, which in turn influences the crystal size distribution which is in fact obtained. When the production of azo pigments is carried out in a batch-wise manner, the coupling component of the coupling reaction is usually suspended in the liquid reaction medium, and the formed azo pigment, which is only slightly soluble in the reaction medium, will precipitate.
In a continuous process comprising a coupling reaction, one of the most important process conditions is the concentration of diazonium reactant, and it would be a great advantage to be able to maintain this concentration substantially non-fluctuating despite a variation in the rate of introduction of the starting materials. It would also be desirable to be able to control the concentration of diazonium reactant in the reactor outlet, i.e. to maintain the concentration at a chosen value.
Hitherto, the diazonium reactant has most frequently been detected using a spot test in which a coupling component (a reagent) of a coupling reaction is applied to a conventional filter paper. A sample of the reaction medium to be tested is applied to the filter paper at a small distance from the applied sample of coupling component. The sample will spread on the paper and eventually reach the region containing the coupling component (the reagent). If a diazonium compound is present in the sample, a strongly coloured reaction product, an azo compound, will be formed and will be clearly visible (the higher the concentration of diazonium compound, the stronger the colour formed on the filter paper). Besides the very qualitative nature of the test, this detection test is tedious and tiresome and cannot be used for anything but a slow, inaccurate manual control of the addition (or rate of addition) of diazonium reactant to the reactor.
It should be noted that the industrial production of azo compounds generally takes place in reaction vessels having a volume of 40 to 80 m.sup.3. This large scale production further emphasizes the need for a fast and reliable method for monitoring.
As a consequence, one object of the invention is to provide a method, preferably a method for utilization in an on-line automation system for optimization and control, which quantitatively monitors a reactant of a coupling reaction in which an azo colourant is formed.
It is essential (i) that the method entails measurements having short response times so that measurement is made on-line or is substantially a real-time measurement with respect to the actual process in the liquid medium, (ii) that the method can be carried out in situ so as to avoid any of the usual disadvantages connected with sampling, and (iii) that the method is suitable for use under a variety of conditions Also, it is most desirable that the method has a high degree of simplicity and that the equipment used in the method can be operated continuously for long periods without requiring maintenance.
In the search for a satisfactory method, it has been found that the well-known FIA methods cannot be applied to the reaction process decribed above for the following reasons:
sampling from a reaction medium comprising liquid and solid phases is extremely difficult, the sample may not be representative of the bulk reaction medium, and any solid phase present in the sample, as is the case in azo pigment production, may lead to blockage of the the sampling system;
the coupling reaction taking place in the reaction medium proceeds in the FIA piping and detection means, thereby leading to precipitation of the azo pigment in question, leading in turn to unreliable analysis results and eventually to blocking of the FIA system;
thus the FIA system is inadequate for monitoring in an on-line automation system, since, at best, very frequent maintenance is essential.